Error: "553 Mail from x.x.x.x not allowed" when sending email to Yahoo
Open proxies and open relays are a very common source of spam. Yahoo uses a variety of techniques to detect these, and Yahoo doesn't accept SMTP connections from either open relays or open proxies. Your connection may be rejected if the server that you use for sending email is "open relay" or "open proxy."
These types of email servers can easily be tricked into sending large amounts of unsolicited email. Many email services refuse mail from such servers, until the server has been fixed to stop relaying spam to innocent users.
Correct the problem
To fix this, you should contact the system administrator of your mail server (usually "firstname.lastname@example.org") and forward them a copy of the bounced email you received. Otherwise, you should contact their technical support and alert them of the problem.
About Yahoo testing for open relays and open proxies
To determine whether an IP address is an open relay or open proxy, a series of tests need to be performed on that IP address.
Yahoo actively performs tests for open relays and open proxies, but only after that IP address has been identified as a source of significant suspicious inbound traffic to the Yahoo network.
In other words, Yahoo only makes reactive tests triggered on inbound traffic from that IP address, and proactive tests are never performed by Yahoo.
Yahoo strives to ensure that any testing performed as a consequence of the inbound traffic triggers have the least impact possible on the tested address.
Once an IP address has been tested, no further tests are performed for at least three days regardless of the outcome of the tests.
Actions based on test results
If an IP address is determined to be an open relay or open proxy, Yahoo reserves the right to reject all SMTP traffic from that IP address for a minimum period of 60 days.
Source address of tests
All tests run by Yahoo are run on readily identifiable servers that have forward and reverse DNS entries ending in "yahoo.com." Any email addresses or servers used as part of the test are identified with "yahoo.com" DNS entries or "yahoo.com" email addresses.